The requirement for hover stability and control of vertical takeoff and landing (VTOL) rotorcraft other than helicoptersxe2x80x94which are limited in forward speed capabilityxe2x80x94has normally produced solutions which are far from ideal; most being either more complex than a helicopter or compromised in some fashion.
Ideally, just two propellers or fans would be all that was required for providing lift and control in hover. However, past VTOL aircraft have not been able to hover in a stable manner and under full control without additional reactive devices, these devices primarily enabling control about the axis joining the two fans.
For their stability and control in hover, all past and present VTOL rotorcraft have employedxe2x80x94in their construction or proposalxe2x80x94either: cyclic blade-pitch controls (helicopters and tilt-rotors); more than two propellers or fans; vanes in the propeller/fan slipstreams; or some other secondary, reactive device in addition to the main lifting propellers/fans.
Cyclic pitch control is not suitable for small, high speed fans, and so is not-conducive to VTOL aircraft with small footprints. Moreover, this solution results in a duplicity of intricate mechanics when applied to more than one rotor.
Aircraft using more than two fans have mechanically complex and heavy drive-trains, and their configurations are usually compromised or restricted by the additional devices. These deficiencies are compounded when the aircraft is intended to transition to airplane mode: either all the fans/propellers must be made to tilt, or some become excess weight and drag.
Hover stability and control using just vanes or control surfaces in the propeller/fan slipstreams have been marginalized by the difficulty in obtaining sufficient control moments, since their effectiveness depends on their vertical distance from the aircraft center of gravity and so restrictsxe2x80x94or is restricted byxe2x80x94the aircraft configuration. For instance, there may be reduced vane effectiveness in ground proximity.
By utilizing the gyroscopic properties of dual, counter-rotating propellers or fans, the present invention provides aircraft control without the need for cyclic controls or additional reactive devices.
Tilting the two propellers directly towards or away from each other creates gyroscopic and propeller-torque moments about the axis perpendicular to the tilt and mean spin axes, and so provides the required aircraft control about that axis. Specifically, the propeller axes are made to tilt within a common plane as necessary, in opposite directions by an equal amount and rate. The control method is therefore hence referred to as opposed tilting. The unbalanced gyroscopic moments are a result of the tilt rate, and the propeller-torque moments are due to the tilt angle from the aircraft vertical. These effects are independent of the vertical placement of the aircraft center of gravity.
For full aircraft control the propellers may also tilt in a direction perpendicular to the opposed tilting direction, providing both horizontal motion and yaw control. They also may be tilted collectively as well as oppositely in the opposed tilting direction. In general, the resulting combined tilting is referred to as oblique tilting.